1. Field of the Invention
The present invention relates to water-based surface treating agent that forms a chemical layer on the surfaces of copper and copper alloys and is suited for use as a preflux for copper circuit on rigid printed wiring boards and on flexible printed wiring boards.
2. Prior Art
Surface treatment methods of forming a chemical layer of an alkylimidazole compound having a long-chain alkyl group at the 2-position on the surfaces of copper or copper alloys have been disclosed in Japanese Patent Publications Nos. 17046/1971, 11454/1973, 25621/1973, 1983/1974, 26183/1974, 22545/1983, 41988/1986 and in Japanese Laid-Open Patent Publication No. 90492/1986.
Surface treatment methods of forming a chemical layer of an imidazole compound substituted with an aryl group at the 2-position on the surfaces of copper or copper alloys have been disclosed in Japanese Laid-Open Patent Publications Nos. 202780/1992 and 206681/1992.
As for the surface treatment methods of forming a chemical layer of a benzimidazole-type compound on the surfaces of copper or copper alloys, a surface treatment method using a 5-methylbenzimidazole has been disclosed in Japanese Laid-Open Patent Publication No. 501281/1983, and surface treatment methods using a 2-alkylbenzimidazole compound, a 2-arylbenzimidazole compound, a 2-aralkylbenzimidazole compound and a 2-mercaptoalkylbenzimidazole compound have been disclosed in Japanese Laid-Open Patent Publications Nos. 124395/1991, 236478/1991, 72072/1992, 80375/1982, 99285/1992, 157174/1992, 165083/1992, 173983/1992, 183874/1992, 202780/1992, 206681/1992, 218679/1992, 25407/1993, 93280/1993, 93281/1993, 156475/1993, 163585/1993, 175643/1993, 186880/1993, 186888/1993, 202492/1993, 230674/1993, 237688/1993, 263275/1993, 287562/1993, 291729/1993, 287563/1993 and 291729/1993.
Furthermore, methods of preventing copper or copper alloys from rusting by using a 2-mercaptobenzimidazole have been disclose d in Japanese Laid-Open Patent Publications Nos. 83157/1980, 77600/1987 and 118598/1988.
In recent years, surface mount device (SMD) have in many cases been mounted on the surfaces of the printed wiring boards (PWBs). Therefore, the PWBs are frequently subjected to high temperatures from such needs as temporary mounting of the SMDs, mounting of the SMDs on both surfaces, and mounting of the SMDs and discrete parts in a mixed manner.
Due to the poor heat-resistance of the imidazole compounds having a long-chain alkyl group at the 2-position, PWBs treated with these imidazole compounds show poor solderability after being subjected to high temperatures.
As for the surface treating method using the 5-methylbenzimidazole disclosed in Japanese Laid-Open Patent Publication No. 501281/1983, this compound dissolves in the water so easily that it is not allowed to form a desired coating having a thickness of 0.08 .mu.m or greater on the surfaces of the copper. Therefore, the underlying copper is not sufficiently protected from oxidation under high temperature conditions, and copper oxide that is formed hinders the soldering.
As for the methods of preserving solderability by using the 2-alkylbenzimidazole compound, 2-arylbenzimidazole compound, 2-aralkylbenzimidazole compound and 2-mercaptoalkylbenzimidazole compound, a chemical layer having good heat resistance can be formed on the surface of the copper; however, the following problems must be solved for practical use.
That is, the benzimidazole-type compounds dissolve little in the water and easily undergo crystallization as the pH of t he prepared treating solution increases or as the treating solution vaporizes. The solid benzimidazole-type compounds that have once crystallized are dissolved again with difficulty despite the efforts of lowering the pH of the treating solution by adding acids or by replenishing the water that has vaporized.
When the benzimidazole-type compounds crystallize or precipitate in the manufacturing process of treating the surfaces of copper of PWB. PWB fabricator is obliged to wipe off the benzimidazole-type compounds that have adhered on the machine or to clean up the machine. Moreover, the solid benzimidazole-type compounds adhered on the surfaces of the PWB seriously deprive them of commercial values. Removal of the adhered benzimidazole-type compounds requires additional repair-work which is much of a problem. The heat-resistant e of the chemical layer of these benzimidazole-type compounds are good compared with those of alkylimidazole compounds; however, much more technical improvements on heat-resistance, solderability (solder flow-up and spreadability of solder paste), and ease of handling are still required.
According to the methods of preserving solderability by using the 2-mercaptobenzimidazole described in Japanese Laid-Open Patent Publications Nos. 83157/1980 and 77600/1987, the 2-mercaptobenzimidazole is dissolved in an organic solvent such as methanol, applied onto the PWBs and is dried leaving, however, there are problems such as adversely affecting the human body due to the use of an organic solvent and from the standpoint of maintaining safety in the factory. As for the method described in Japanese Laid-Open Patent Publication No. 118598/1988, the dip processing of about three hours is necessary for forming a thin chemical layer of 2-mercaptobenzimidazole, making itself not suited for practical use under the circumstances of PCB business where high productivity and high-speed processing are required.
Japanese Laid-Open Patent Publication No. 206681/1992 discloses a method of forming chemical layer on the surfaces of copper by dipping PCBs in an aqueous solution which contains an imidazole compound substituted with an aryl group at the 2-position and higher fatty acids or higher fatty acid compounds such as ammonium salts or amine salts of higher fatty acids. As the imidazole compounds substituted with an aryl group at 2-position, there have been exemplified a 2-phenylimidazole, a 2-tolylimidazole, a 2-phenyl-4-methylimidazole, a 2-phenyl-1-benzylimidazole, a 2,4,5-triphenylimidazole.
The invention disclosed in Japanese Laid-Open Patent Publication No. 206681/1992 contains, as essential components, an imidazole compound substituted with an aryl group at the 2-position and higher fatty acids or higher fatty acid compounds such as ammonium salts or amine salts of higher fatty acids. According to the invention disclosed therein, a chemical coating having excellent heat resistance is obtained by making a higher fatty acid present in the imidazole compound substituted with an aryl group at the second position that is little capable of forming a coating.
However, in order to form the chemical layer and to exhibit its performances and water repellency, the above invention must use a higher fatty acid compound with 12 to 22 carbon atoms as an essential component. Without using the higher fatty acid, it is not allowed for the above invention to form a chemical layer. To dissolve the higher fatty acid compound in the water, the solution must usually be alkaline. In order to dissolve the imidazole compound substituted with an aryl group at the 2-position in the water, however, the solution must be rendered acidic. That is, to put the above invention into practical use, there arises a difficulty in preparing an aqueous solution in which are present both the imidazole compound having an aryl group at the 2-position and the higher fatty acid compound. To render the two components dissolved, a mixture solution must be prepared by adding water-soluble organic solvents such as methanol and acetone in addition to the water.
However, use of such organic solvents causes problems such as adversely affecting the human body and from the standpoint of maintaining safety in the factory. Besides, the vaporization of the organic solvent during the use causes the composition of the treating solution to change and brings about problems in that the imidazole compound having an aryl group at the 2-position and higher fatty acid compounds which are active ingredients are liable to be precipitated.
To protect the copper circuit on the PWBs by using a chemical layer under the above-mentioned circumstances, therefore, it has been desired to provide a preflux having excellent properties such as excellent heat resistance and solderability to meet the surface-mounting technology of PCB industry, without permitting solid matters to precipitate in the treating solution during the use or enabling the solid matters to be easily dissolved again if they were precipitated.